1. Field of the Invention
The present invention relates to a shaft encoder. More particularly, the invention relates to a fiber optic converter for such a shaft encoder.
2. Prior Art
Shaft encoders are electromechanical devices which provide electronic representations of the variable properties of a rotating shaft. By properly coupling the input shaft of the encoder, and operating the encoder in conjunction with suitable electronics, the output from the encoder may be made to represent angular or linear position, speed of rotation, direction of rotation, and amount of rotation. Moreover, incremental shaft encoders may provide high-level and/or low-level logic outputs having both single-ended and differential (i.e., complementary pair) signals. A standard connection between an incremental encoder and a shaft control system may be completed using six wires for the three signals (using differential signaling), and another pair of wires for the power supply. However, the problem with these encoders is that in many applications the signal wires are often bundled or cabled along with AC power cables, high-frequency signal cables, and/or other cables that contain high levels of electromagnetic fields. These fields may generate interferences with the encoder signals. Moreover, the encoder signals, if carried for distances exceeding several meters, are susceptible to interference from lightning induced voltages.
To alleviate the problems associated with Electro Magnetic Interference (EMI), differential signaling may be used where two logic signals are separated by 180 degrees in phase. However, even with differential signaling, other problems arise due to the non-isolated nature of these signals, including ground bounce, overshoot, ringing, and ground loops, each generating a unique noise signature and interference signature. Furthermore, the encoder signal may not be completely immune from electromagnetic interferences, especially when the line length becomes longer than a few feet and when these events are generated by lightning.